Question

I’m compiling my code via the following command:

icc -ltbb test.cxx -o test

Then when I run the program:

time ./mp6 100 > output.modified
Floating exception
4.871u 0.405s 0:05.28 99.8%     0+0k 0+0io 0pf+0w

I get a “Floating exception”. This following is code in C++ that I had before the exception and after:

// before
if (j < E[i]) {
   temp += foo(0, trr[i], ex[i+j*N]);
}

// after
temp += (j < E[i])*foo(0, trr[i], ex[i+j*N]);

This is boolean algebra… so (j < E[i]) is either going to be a 0 or a 1 so the multiplication would result either in 0 or the foo() result. I don’t see why this would cause a floating exception.
This is what foo() does:

int foo(int s, int t, int e) {
    switch(s % 4) {
        case 0:
            return abs(t - e)/e;
        case 1:
            return (t == e) ? 0 : 1;
        case 2:
            return (t < e) ? 5 : (t - e)/t;
        case 3:
            return abs(t - e)/t;
    }
    return 0;
}

foo() isn’t a function I wrote so I’m not too sure as to what it does… but I don’t think the problem is with the function foo(). Is there something about boolean algebra that I don’t understand or something that works differently in C++ than I know of? Any ideas why this causes an exception?

Thanks,
Hristo

Answer 1

You are almost certainly dividing by zero in foo.

A simple program of

int main()
{
    int bad = 0;
    return 25/bad;
}

also prints

Floating point exception

on my system.

So, you should check whether e is 0 when s % 4 is zero, or whether t is 0 when s % 4 is 2 or 3. Then return whatever value makes sense for your situation instead of trying to divide by zero.

---

@hristo: C++ will still evaluate the right-hand-side of a multiplication even if the left-hand-side is zero. It doesn’t matter that the result should be zero; it matters that foo was called and evaluated and caused an error.

Sample source:

#include <iostream>
int maybe_cause_exception(bool cause_it)
{
    int divisor = cause_it ? 0 : 10;
    return 10 / divisor;
}

int main()
{
    std::cout << "Do not raise exception: " << maybe_cause_exception(false) << std::endl;

    int x = 0;

    std::cout << "Before 'if' statement..." << std::endl;

    if(x)
    {
        std::cout << "Inside if: " << maybe_cause_exception(true) << std::endl;
    }

    std::cout << "Past 'if' statement." << std::endl;

    std::cout << "Cause exception: " << x * maybe_cause_exception(true) << std::endl;

    return 0;
}

Output:

Do not raise exception: 1

Before 'if' statement...

Past 'if' statement.

Floating point exception